The woman who was wed for 18 years to Republican White House hopeful Newt Gingrich said in an interview airing Thursday that her ex-husband asked for an "open marriage" after carrying on a six-year affair with an aide

WASHINGTON — The woman who was wed for 18 years to Republican White House hopeful Newt Gingrich said in an interview airing Thursday that her ex-husband asked for an “open marriage” after carrying on a six-year affair with an aide.

Her explosive revelation comes just two days before the critically important primary in South Carolina, in America’s Bible Belt, and could dent Gingrich’s surge as he seeks to close the gap with frontrunner Mitt Romney in the race for the party nomination.

In an interview detailing the unraveling of her marriage, Marianne Gingrich said the former House speaker sought a marital arrangement that would have allowed him to keep his mistress while remaining married to her.

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“He wanted an open marriage and I refused,” Marianne Gingrich told ABC News’s “Nightline” program, in a video clip airing in advance of Thursday’s broadcast.

She said Gingrich admitted to a six-year affair with congressional aide Callista Bisek — now Callista Gingrich — whom he wed after her marriage to the Georgia lawmaker fell apart.

Marianne Gingrich was the second woman to be married to Gingrich, one of the remaining presidential contenders who, while House speaker in the 1990s once was among Washington’s most powerful figures.

She said it was the second time that Gingrich betrayed the woman he was married to in order to marry a lover, noting that she herself started out as Gingrich’s paramour, while he was married to first wife Jackie Battley Gingrich.

Marianne Gingrich described for ABC her “shock” at her then-husband’s behavior, including how she said she learned he conducted his affair with Callista “in my bedroom in our apartment in Washington.”

“He always called me at night,” she recalled, “and always ended with ’I love you.’ Well, she [Callista] was listening.”

She added that Newt filed divorce papers a short time after she received a diagnosis of multiple sclerosis — even though her doctors warned it was imperative that she not be placed under undue stress.

Gingrich appeared on NBC television early Thursday and was asked about his feelings about his former wife’s decision to go public.

“I’m not going to say anything negative about Marianne,” he said.

But he said the expose was “completely wrong,” and accused ABC of “intruding into family things that are a decade — more than a decade — old.”

He added that he has decided to allow his two adult daughters from his first marriage to handle his refutations about Marianne Gingrich’s interview.

“I’m not going to comment about it. I’m focused on the big issues that concern the American people,” he said.

He added that that voters “will have to judge me,” based upon a fuller picture of his life and accomplishments.

“I’m a 68-year-old grandfather,” Gingrich said.

“See how close I am to my wife, Callista. And how close I am to my daughters and son-in-laws, my two grandchildren, they’ll have to make their mind up.”